Largely left to their own devices, human stem cells knitted themselves into tissue with a multitude of brain structures and specialized cadres of neurons in a form reminiscent of the brain of a nine-week-old fetus, scientists report August 28 in Nature.

The tissue doesn’t approach the dizzying complexity of the human brain. Yet these tiny neural balls, each no bigger than a BB pellet, represent the most complex brain structure grown in a lab to date, researchers say. The new work could provide an unprecedented window into the early s